How is the altitude measured for an Aircraft
They determine altitude by measuring air pressure. As altitude increases, air pressure decreases. This is because the density of air is lower (thinner) at high altitudes. It exerts less pressure on Earth below.
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The altimeter measures the height of an aircraft above a fixed level. The instrument senses this by taking the ambient air pressure from the static port. That air is plumbed through the back of the panel and into the back case of the altimeter. Inside the altimeter is a sealed disc called an aneroid, or bellows.
The pilot of an aircraft
needs to know its altitude and there are several ways we could measure the
altitude of an airplane. Radar might be used to measure the plane’s distance
above the ground. Global Positioning System satellite signals can determine the
plane’s position, including its altitude, in three dimensional space. These and
some other possible methods of altitude determination depend on the operation
of one or more electrical systems, and while we may want to have such an instrument on our airplane,
we also are required to have an “altimeter” that does not depend
on batteries or generators for its operation. Further, the altitude that the
pilot needs to know is the height above sea level. The obvious solution
is to use our knowledge
that the pressure
varies in a fairly dependable fashion with altitude.
If we know how pressure
varies with altitude then we can measure that pressure and determine the
altitude above a sea level reference
point. In other words, if we measure
the pressure as 836 pounds per square foot we can look in
the standard atmosphere table and find that we should be at an altitude of
23,000 ft. So all we need to do is build a simple mechanical barometer and
calibrate its dial so it reads in units of altitude rather than pressure. As
the measured pressure decreases, the indicated altitude increases in accord
with the standard atmosphere model. This is, in fact, how “simple” altimeters such as those
sometimes used in cars or bikes or even “ultra-lite” aircraft. A barometer measures the air
pressure and on some type of dial or scale, instead of pressure units, the
equivalent altitudes are indicated.
The “simple” altimeter,
however, might not be quite accurate enough for most flying because of the
variations in atmospheric pressure with weather system
changes. The simple
altimeter would base its reading
on the assumption that the pressure
at sea level is 2116 psf . If, however,
we are in an area of “high” pressure, the altimeter of an airplane
sitting at sea level would sense the higher than standard
pressure and indicate
an altitude somewhat
below sea level. Conversely,
in the vicinity of a low pressure
atmospheric system the altimeter would
read an altitude
higher than the actual value.
If this error was only a few feet it might not matter,
but in reality it could result in errors of several hundred
feet in altitude readings. This could lead to disaster in bad weather
when a pilot has to rely on the altimeter to ensure that the plane clears mountain
peaks or approaches the runway at the right
altitude. Hence, all aircraft today
use “sensitive” altimeters that allow the pilot to adjust
the instrument for changes in pressure due to atmospheric weather patterns.
The sensitive altimeter, shown in the next figure, has
a knob that can be turned to adjust the readout of the instrument for non-standard sea level pressures. This can be used in two different
ways in flight. When the aircraft is sitting on the
ground at an airport the pilot can simply adjust
the knob until
the altimeter reads
the known altitude
of the airport. In flight, the pilot can listen to weather report
updates from nearby airports, reports that will include the current sea level equivalent barometric pressure, and turn the knob until the numbers
in a small window on the altimeter
face agree with the stated
pressure. These readings are usually given in units of millimeters of mercury
where 29.92 is sea level standard. Adjusting the reading in the window to a higher pressure
will result in a decrease
in the altimeter reading and adjusting it lower will increase the
altitude indication. With proper and timely use of this adjustment a good
altimeter should be accurate within about 50 feet.
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Figure 1.4: A Typical Mechanical Sensitive Altimeter |
It should be noted
that we could
also use density
to define our altitude and, in fact,
this might prove
more meaningful in terms
of relating to changes in an airplane’s performance at various
flight altitudes because
engine thrust and power are known
to be functions of density
and the aerodynamic lift and drag are also functions
of density. However,
to “measure” density would
require measurement of both pressure and temperature. This could introduce more
error into our use of the standard
atmosphere for altitude determination than the use of pressure alone because
temperature variation is much more subject
to non-standard behavior
than that of pressure. On the other hand, we do sometimes
find it valuable to calculate our “density altitude” when looking at a
plane’s ability to take-off in a given ground distance.
If we are at an airport
which is at an altitude
of, lets say, 4000 ft and the temperature is higher than the 44.74oF predicted
by the standard atmosphere (as it probably
would be in the summer)
we would find that the airplane behaves
as if it is at a higher altitude and will take a
longer distance to become airborne than it should at 4000 ft. Pilots use either
a circular slide rule type calculator or a special electronic calculator to
take the measured real temperature and combine it with the pressure altitude to
find the “density altitude”, and this can be used to estimate the extra takeoff
distance needed relative to standard conditions.
Some may wonder why we can’t
simply use temperature to find our altitude. After all, wasn’t one of our basic
assumptions that in the Troposphere, temperature dropped linearly with
altitude? Wouldn’t it be really easy to stick a thermometer out the window and
compare its reading with a standard atmosphere chart to find our altitude?
Of course, once we are above the |
Best to know;